Various boring devices have been proposed over the years; however, recently there is a demand for boring not only to form a usual circular hole but also to form a non-circular hole. For example, in the boring device of JP H11-179605 A, a tool that is provided at the distal end of a tool holder extending in the form of a bar is protruded in the radial direction using a piezoelectric element. Here, boring is performed to form a non-circular hole by rotating the tool holder about the axis while adjusting the protrusion length of the tool using the piezoelectric element.